A tunable impedance presents an impedance value, to a larger circuit system, that is tunable in response to a control signal. Tunable impedances find use in a variety of circuits, including voltage-controlled oscillators, continuous-time filters and others. Such circuits occur in a myriad of applications, such as communications, processing, etc.
FIG. 1 depicts an embodiment of an oscillator circuit 20 having a tunable inductor-capacitor combination 24. The oscillator circuit includes a pair of cross-coupled transistors 28, 30 and a voltage-controlled current source 32. The tunable inductor-capacitor combination 24 includes an inductor L1 and a tunable capacitor CT connected in parallel across respective drains of the cross-coupled pair 28, 30. The inductor L1 is connected to an upper power supply VDD at a center tap. The cross-coupled pair 28 is supplied with a current from the voltage-controlled current source 32. The current source 32 includes a plurality of transistors MA-1 to MA-X (collectively, MA) selectively connected, through a plurality of switches SA-1 to SA-X and SB-1 to SB-X (collectively, SA, SB), to a reference current source I1 through a current-mirror transistor configuration 36. A plurality of control signals supplied to the switches SA, SB connecting the transistors MA to the current mirror 36 control the total current supplied by the voltage-controlled current source 32 to the cross-coupled pair 28, 30.
In operation, the cross-coupled transistors 28, 30 induce an oscillation, across the inductor-capacitor combination 24, having a frequency controlled by the impedance values of the inductor L1 and tunable capacitor CT. In FIG. 1, the inductor L1 has a fixed inductance value and the capacitor CT a selectable capacitance value, and the oscillation frequency can thus be controlled by selecting the capacitance value of the tunable capacitor CT.
FIG. 2 depicts an embodiment of a tunable capacitor 40 that is used to implement the capacitor CT of the oscillator 20. The tunable capacitor 40 includes a fixed capacitance CA in parallel with a plurality of selectable capacitance branches 44A-44X. Each selectable capacitance branch 44-1 to 44-X (collectively, 44) includes first and second capacitances CB-1 to CB-X and CC-1 to CC-X (collectively, CB, CC) connected in series with a centrally situated selection switch SC-1 to SC-X (collectively, SC). In operation, the capacitance value of the tunable capacitance 40 can be controlled by selectively enabling and disabling the selection switches SC of the selectable capacitance branches 44. For example, with none of the selection switches SC enabled, the tunable capacitance 40 can have a value equal to that of the fixed capacitance CA. Enabling of a particular selection switch SC can then bring the series combination of the respective first and second capacitances CB, CC in parallel with the fixed capacitance CA, thereby modifying the overall tunable capacitance value.
One problem encountered in realizing the tunable capacitor structure 40 of FIG. 2 is distortion introduced by the selection switch SC into the selectable capacitance branches 44, the tunable capacitor 40 and containing circuits. FIG. 3 depicts an embodiment of a transistor realization of the selectable capacitance branch 44. In FIG. 3, the selection switch SC can be implemented by a single n-channel metal-oxide-semiconductor (NMOS) transistor MB. A control circuit can include a pair of inverters INV1, INV2 to provide drive signals to enable and disable the switch transistor MB. A first drive signal can be provided to the gate of the transistor MB, and a second drive signal can be provided to the source and drain of the transistor MB through a pair of corresponding impedances RA, RB.
Distortion can be introduced by the selection switch SC in both its on and off states. When the switch SC is on, the embodiment of FIG. 3 can present the on-resistance of the transistor MB between the first and second capacitances CB, CC of the capacitance branch 44. The presence of the on-resistance can introduce distortion as the on-resistance can be a function of the differential signal appearing across the tunable capacitance 40. This transistor on-resistance can typically depend inversely on the width-to-length ratio of the transistor. The distortion caused by the on-resistance of the selection transistor MB can thus be reduced by increasing the width-to-length ratio of the transistor, thereby reducing the on-resistance, the voltage appearing across on-resistance, and the resulting distortion.
However, increasing the width of the switch transistor MB, to reduce the on-resistance distortion contribution in the on state, may adversely affect the distortion contribution of the switch in the off state. In a typical complimentary metal-oxide-semiconductor CMOS) process, the drain-to-body and source-to-body paths of a transistor are PN junctions. FIG. 4 depicts a circuit modeling the selectable capacitance branch 44 of FIG. 3 and connected portions of an embodiment 20b of the oscillator circuit of FIG. 1, and illustrating the drain-to-body and source-to-body PN junctions as two diodes D1, D2. Although these diodes D1, D2 can be illustrated in FIG. 4 as separate components for explanation purposes, they are inherent to and part of the switch transistor MB, as indicated by the use of dashed-line connections. When the switch transistor MB is off, these junction diodes D1, D2 can be reverse biased, and present associated non-linear parasitic capacitances between the drain and body and between the source and body (i.e., source-to-body and drain-to-body parasitic capacitances CSB, CDB, as illustrated in FIGS. 10B, 11B, and 12B and discussed below). Any differential voltage present across the tunable capacitance in the off state can appear at the junction capacitances, and resulting corresponding current can be injected into connected portions of the oscillator circuit 20b, thereby introducing distortion. The size of this distortion current can increase with the size of the junction capacitances, which in turn can depend on the width of the transistor MB. Thus, although increasing the width of the switch transistor MB may reduce the distortion in the on state, it may increase the distortion in the off state.
A need therefore exists for impedance selection circuits, tunable impedances, and containing circuits that have low distortion in both on and off impedance selection states.